Recent years, with the reduction in size and weight of electronic equipment, the switching power supplies, that are smaller and lighter and can efficiently output power, have been widely used. In a switching power supply, when electric insulation is necessary between the input side and the output side, a transformer is used. As a switching power supply using a transformer, a flyback switching power supply, with which a high power output voltage can be obtained, is known.
In the flyback switching power supply, a primary-side winding of the transformer and a switching element are series-connected, and a direct-current voltage obtained by rectifying and smoothing an alternating-current voltage of the commercial power supply voltage is applied to the series circuit of them, and current flows through the primary-side winding of the transformer when the switching element is turned on and off. When the switching element is turned on, the current flowing through the primary-side winding of the transformer accumulates energy in a core of the transformer, and when the switching element is turned off, the accumulated energy is released from the secondary side winding of the transfer such that the output current is charged in a capacitor via a diode, and thereby, a direct-current output voltage is generated.
Thus, in the flyback switching power supply, the operation changes according to switching between on and off of the switching element, and the output voltage can be controlled by varying the ratio of on-period and off-period. Further, the output voltage also changes depending on the turn ratio or characteristics of the transformer.
On the other hand, when the electric insulation is not necessary between the input side and the output side, a chopper type switching power supply using a choke coil in place of a transformer is used. Generally, as a magnetic material for a core of the transformer or the choke coil, ferrite having low loss and high efficiency is used. However, since ferrite is apt to be magnetically saturated, when the current of a winding wound around a core exceeds a certain value, the core is saturated and the magnetic characteristic becomes lower. In order to avoid this, it is necessary to form a gap in the core, but a problem of leakage of magnetic flux arises in this case.
As a related technology, Japanese Patent Application Publication JP-P2004-320917A discloses a switching power supply device that prevents failure of circuit elements due to overcurrent of power supply output with a circuit having a simple configuration, and contributes to downsizing and cost reduction of the device. The switching power supply device includes a current detection circuit for detecting a current flowing in a main switch element and outputting the detection result to a current detection terminal of a control IC, and the control IC has a pulse-by-pulse overcurrent protection circuit for turning off the main switch element when the current flowing through the main switch element exceeds a predetermined threshold value.
Furthermore, the switching power supply device includes a power supply output detection circuit for detecting the output voltage of the power supply, and a switching frequency setting circuit for setting a switching frequency of the main switch element. When the output voltage of the power supply becomes lower than a preset value, a part of the current flowing in the switching frequency setting circuit is bypassed to the current detection circuit and superimposed to the current therein, and the control IC detects the superposed current at the current detection terminal and operates to suppress the current flowing through the main switch element. At the same time, the switching power supply device reduces the switching frequency provided by the switching frequency setting circuit to perform over current protection operation of the power supply output.
JP-P2004-320917A discloses that downsizing of components such as a rectifying element and transformer can be realized, but does not disclose that magnetic materials other than ferrite may be used as a core of the transformer.
Further, Japanese Patent Publication JP-P3642398 (JP-P2001-75659A) discloses that, in a power supply control method of controlling the output of the power supply, the maximum rated output that permits the overload condition exceeding the rated output in a permitted time is variably set according to a temperature.
Furthermore, Japanese Patent Application Publication JP-P2003-284330A discloses a control device that controls a direct-current converter with reference to the size of a switching element in an actual use situation. The control device includes detecting means for detecting an input voltage to the direct-current converter, determining means for determining the maximum rated current that indicates the maximum value of the operating current of the direct-current converter according to the voltage level of the detected input voltage so as to suppress the current when the input voltage becomes lower, and suppressing means for suppressing output current when the output current of the direct-current converter exceeds the maximum rated current.
However, JP-P3642398 or JP-P2003-284330A does not disclose that magnetic materials other than ferrite may be used as a core of the transformer.